Many electronic imagers produce a colored fringe around very bright areas in the image. The fringe tends to be magenta or blue although the fringe color may not be limited to these colors. Bright color fringes are particularly prevalent and noticeable in areas where the saturated area is surrounded by a dark area. The problem is common around backlit trees or around the perimeter of specular reflections in jewelry or silverware. The cause is believed to be related to cross-talk due to light piping in the array or due to ray angle.
Cross-talk due to ray angle occurs when a light ray falls on the imager surface at an angle, passes through a CFA patch, and falls on an adjacent pixel associated with a different color CFA patch. Ray angle cross-talk is related to the ratio of the height of the CFA patch over the active area to the pixel pitch. The larger the ratio the larger effect ray angle will have on cross-talk. Cross-talk tends to increase as the pixel pitch gets smaller because the space between the CFA and the active area tends to be larger in proportion to the pixel pitch.
Cross-talk can also be caused by light piping. Light piping occurs when light is trapped in the transparent layer between the imager top surface and the active area below. Light passes through the top surface of the imager, passes through the CFA and other transmissive layers and reaches the photo active area and is reflected toward the top surface of the imager. There is usually some scatter at this reflection. Some of the scattered light hits the top surface layer at an angle large enough that the reflection is substantial. The reflectivity of the surface increases with angle to the normal until critical angle is reached, and then all of the light is reflected back to the active area because of total internal reflection. Some light will be absorbed between the CFA and the active surface, some light will be absorbed by the CFA and some light will be absorbed at the active surface.
This sequence is repeated a number of times until the effect of the scattered light becomes insignificant due to absorption.
Two patents that describe using a brush to “crawl” around and object and change the edges of an image are: EP0439333B1 and EP0441499B1. These patents do not describe how to locate the type of fringe in question and they do not describe how to isolate and mask the fringe.
In this document, arrays are two-dimensional and are in some ways treated as an image. There is exact correspondence between rows and columns in an image and the rows and columns in an array. Similarly, a map is a two-dimensional array of values with exact correspondence between the rows and columns of the map and the rows and columns of the image. The map contains numerically encoded logical information about the pixels in the corresponding image. For example, a value in a map might be set to zero if the pixel with the corresponding row and column is less than a first threshold, the value might be set to one if the pixel with the corresponding row and column is within a range of values, and it could be set to two if the pixel with the corresponding row and column is above a second threshold.
In this document, the meaning of a difference between parameters is not limited to an arithmetic difference. Difference can also mean a difference between a threshold ratio and the ratio of two numbers, or any other difference that makes one parameter different from another.